A rolling mill typically includes a series of rolling stands which reduce the thickness of a web or strip of material, such as aluminum or steel, in intermediate stages during which the web of material is compressed between rollers of the successive stands. Although the process may be adapted to obtain different products, the rolling process is essentially a deforming process.
The three variables having the largest effect on the resulting product appear to be: (1) the compressive force used to spread, shape or separate the web material; (2) the drive torque which propels the strip through the mill; and (3) the excess heat generated from the mechanical work performed by the rolling operation.
Variables, also referred to as parameters, include both directly controlled variables, such as stand screwdown, and indirectly controlled variables such as compressive force. The directly controlled variables can affect one or more indirectly controlled variables. For example, the greater the amount of screwdown, the closer together are the rolls of the stand, and the greater the compressive force and heat on both the stand and the strip.
While the compressive force is employed to cause the desired deformation, the drive torque and the excess heat affect the deforming process in complex ways. For example, a large amount of drive torque is generated by motors of one stand to drive or thread the strip at a desired speed through the roll bite (entry point) of a subsequent stand. This drive torque deforms the web material which affects the profile and flatness (shape) of the rolled product. Also, the excess heat affects the web material in other ways which can affect the quality of the product.
Additionally, the effects of the drive torque and the excess heat on the strip may interfere with the operation of the rolling mill by causing roll bite threading problems, such as bite refusals, and by causing deterioration of the shape of the threading portion of the strip. These problems result in production delays, lower production rates and poorer quality product. For example, delays can cause the thermal crown to decay both on the work (contact) rolls and also on the backup (support) rolls.
Accurate tandem mill setup procedures are of major importance in avoiding threading problems and in maintaining a high production rate. The primary requirement for controlling the stands during threading is to allow the strip to thread with acceptable tension transients and shape. Strip shape is of overriding importance in threading the first 10 feet of the strip since, unlike the running conditions, there is no tension in the gap between stands. This lack of tension increases the likelihood of buckling with consequent difficulties in feeding the leading edge into the next stand gap.
Solutions to threading problems are made more complex with the requirement for `schedule free` rolling. Schedule free rolling allows any of the possible products produced by the rolling mill to be made one after another without major adjustments to the mill for the transition. For example, in a traditional rolling mill, if the production of one product generates a large amount of excess heat, one would have to delay the production of a second product to allow the heat to dissipate. In one typical instance, this delay could amount to approximately 30 minutes in lost production time.
A mill which avoids excess heat buildup could reduce such delays. Attempts to accomplish schedule free rolling have focused on optimization of three traditional rolling mill quality measures: (1) the number of cobbles (splices in the strip); (2) the coil head gauge (thickness of the strip at the end of the rolling mill where the product is wound into a coil); and (3) the coil head temperature (temperature of the strip at the end of the rolling mill where the product is wound into a coil). Ideally, when a strip reaches the coil head, the strip has no cobbles, but has the desired thickness and the desired temperature. Accomplishment of these ideal measures yields a correct and optimal power balance at the coil head, is more robust in dealing with the presence of transients in strip tension and strip shape as the product is rolled and allows for schedule free rolling.
U.S. Pat. No. 3,820,366 (Smith, Jr.) discusses previous attempts to achieve these ideal measures. Smith counsels the adjustment of the rolling mill variables, such as the amount of stand screwdown, based on the difference in the temperature of a strip being rolled and the average temperature for earlier productions of the particular product.
Another approach, Canadian Pat. No. 1,156,329 (Dekker et al.), selects the rolling mill setup parameters, such as the amount of thickness reduction for each stand, the tension on the strip between stands, and the like, by classifying the web material into one of several standard thickness groups. According to Dekker, each thickness group is defined by entry thickness, exit thickness and exit surface roughness. Once a product is classified, standard setup parameters for that particular thickness group are retrieved and modified according to the difference between the standard thickness group and the particular requirements of the strip to be processed.
As explained by Dekker, the method modifies or adjusts interstand thickness and interstand tension, among other setup parameters, to approach these ideal measures and maintain optimum threadability of the strip. In practice, however, little benefit appears to result from modifying interstand tensions. Accordingly, compensation is achieved traditionally by adjusting the web material thickness after each stand, that is, by adjusting each stand's thickness reduction. However, adapting the standard values of the setup parameters used for the running condition to obtain values for the threading condition cannot adequately address the initial threading conditions unless a set of additional complex calculations specifically designed for the initial threading conditions is used.
A further mill setup procedure used to achieve ideal measures employs horsepower-hour/ton versus thickness reduction curves. However, these curves tend to yield varying thickness reductions, particularly on the first stand which promotes transients, instability and unpredictable threading shape. This type of compensation method is fairly complex to implement, and difficult to understand. It also appears unable to achieve ideal measures.